This invention is concerned with forming an adhesive bond between substrates at least one of which comprises a surface of zinc.
Adhesive compositions have been proposed based on polymerisable acrylic monomers which are polymerisable in absence of oxygen but which are at least substantially stable when stored in presence of oxygen. These adhesives are frequently referred to as anaerobic adhesives. Anaerobic adhesives are fundamentally single part compositions polymerisable primarily through exclusion of oxygen therefrom. They are chiefly known for screw fastening applications. The polymerisation of anaerobic adhesives comprising acrylate or methacrylate monomer is generally initiated by free radical creating compounds e.g. peroxy compounds, contained in the adhesive. Cure of the adhesive composition occurs at room temperature under anaerobic conditions i.e. by the exclusion of oxygen between the substrates to be bonded, for example by screwing a screw into a suitable holder, after applying adhesive composition to one or both components. Such adhesive compositions remain stable at room temperature in the presence of oxygen which inhibits the radically induced polymerisation reaction.
The polymerisation of anaerobic adhesive compositions may be accelerated by accelerator compounds which promote the production of free radicals from the free radical initiator. The accelerator compounds must be stored separately from the free radical initiator before use to prevent premature cure of the composition, and frequently they are supplied as a solution for application to one or both of the surfaces to be bonded as a primer, prior to the application of the adhesive composition. Examples of accelerator compounds include butyraldehyde-aniline condensation products, for example those commercially available from Du Pont under the trade name `Accelerator 808` or from R. T. Vanderbilt Comp. Inc. under the trade name `Vanax 808`.
It has been proposed to employ adhesives based on esters of acrylic or methacrylic acid for use in the field of so called structural adhesives where bonds of high strength and quality in terms of shear, peel, temperature resistance and hydrolysis resistance etc. are required. However, anaerobic adhesives generally fall short of required performance standards in one or more of the required strength or quality aspects of the bonds formed. It has been proposed to employ accelerators in a part of a multipart adhesive composition based on polymerisable acrylic monomers kept separate from the part of the composition containing the free radical initiator, and in this way it is possible to produce adhesives capable of producing bonds to many substrates which are of high strength and quality. For example a polymerisable adhesive composition based on a polymeric component and a monomer mixture comprising 2-hydroxy ethyl methacrylate or 2-hydroxyl propyl methacrylate and a C.sub.1-4 alkyl methacrylate together with a reducing agent and an organic hydroperoxide is described in UK No. 1544006. Various examples of reducing agents are mentioned, including metallic soaps, toluidine compounds, thioamides, thiourea, ethylene thiourea, tetramethylene thiourea and mercaptobenzoimidazole. Such compositions are said to exhibit rapid curing to provide bonds of high tensile shear strength on metal surfaces, for example steel and iron. However, prior to the present invention, we have not been aware of an adhesive composition based on polymerisable acrylic monomers capable of providing bonds of high strength and quality, particularly as exemplified by shear strength, to zinc surfaces for example zinc plated steel.
Among more active accelerators which have been proposed for use with polymerisable acrylic ester based adhesives are compounds containing S- and N- groupings, in particular thioureas for example thiourea, N.N'-dicyclohexyl thiourea and 1-allyl-2-thiourea. The acceleration efficiency of certain thioureas on the radical polymerisation of methyl methacrylate, in solution in presence of certain peroxides and hydroperoxides was investigated in detail by Takaaki Sugimura et al and published in the Journal of Polymer Science at Part A-1 Vol. 4, 2735-2746(1966) (which refers inter alia to Journal of Polymer Science Part A Vol. 3 2935-2945 (1965), and Part A-1 Vol. 4, 2721-2734 (1966)).
It is among objects of this invention to provide improved adhesive bonding to zinc surfaces.